It is known to provide a ground fault circuit interrupter which will open a power or line lead in an electrical system in response to detection of a ground fault. Such circuits are generally quite effective at providing protection in situations such as when a person touches a line lead in equipment or at a socket.
There are, however, many other fault situations which can occur. Two such faults are loss of neutral and loss of ground, which phrases refer to an opening of the neutral or ground leads. Other faults which may occur are that potential may arise between the neutral and ground leads and also reversal of connections, where, for example the line voltage may appear at the neutral lead. All of these faults are indicated by excessive potential arising between neutral and ground and thus, for simplicity they are referred to hereinafter as "loss of neutral or ground".
In an attempt to overcome some of these problems, ground fault circuit interrupters which incorporate loss of neutral protection have been devised. One example is that described in U.S. Pat. No. 4,598,331 (Legatti). In the conventional manner, this circuit includes a differential transformer having opposed primary windings and a secondary winding which is energized when there is an imbalance between the line and neutral leads. When the secondary winding is energized a tripping circuit energizes a circuit breaker to open the line lead. In addition, the neutral and ground leads are interconnected by a supplemental winding on the differential transformer. Power connections between the line lead and both the neutral and ground leads supply a current which passes through the supplemental winding in the event of loss of neutral or ground. On loss of neutral or ground the secondary winding energizes and thereby energizes the circuit breaker to open the line lead.
While it appears that this circuit would be quite effective in most situations, we envisage problems arising in operation under certain circumstances. One problem is that while ground faults and loss of neutral or ground are detected separately, the tripping circuits which are controlled by the detection circuits are intrinsically tied together. For example, the secondary winding is energized both on detection of a ground fault, and also on detection of a loss of neutral or ground. Accordingly, these two different inputs may interfere to prevent corrective action being taken in the case of one or more faults. An extreme example of this is if a ground fault and loss of neutral occur simultaneously, the induced magnetic fields of the differential transformer may cancel, thereby failing to energize the circuit breaker. Secondly, it is clear that if a fault were to occur in the differential transformer itself, then there would be no protection for any of the faults.